A quick example? You have a Clvl 50 barbarian with 80 in Dex and a tower shield that's 49% blocking.
2 * [49 * (80 - 15) ] / (50 * 2 ) = 6370 / 100 = 63.731,85 %.
Compared to the 49% he'd get in D2, he's only half as effective. And it gets worse as your Clvl increases, you need to put about half your stat points into Dex to keep your percentage at around half your shield's blocking %. Twitchthroe is still the only way to boost blocking percentage, and it makes very little difference at higher levels. In the above example it would take the Barb from 25% up to 38%, but he's got 80 dexterity. For a Clvl 50 Sorceress or Necromancer with 25 dexterity, they'd block 3.5% (compared to 44% in D2) with the tower shield, and 5.5% (compared to 69% in D2) with Twitch added in.